1. Field of the Invention
The present invention relates generally to voltage controlled oscillators and their use in phase locking oscillators. More particularly, the present invention pertains to voltage controlled oscillators with reduced incidental frequency modulation and their use in phase locking oscillators.
2. Description of the Related Art
As is well known, frequencies in electronic circuits have a tendency to drift. Therefore, phase locking oscillators are used widely to provide electronic circuits that are highly immune to frequency drift. The output frequency is phase locked to a reference frequency that is commonly crystal controlled.
In phase locking oscillators, an edge-triggered phase detector compares a frequency that is fed back from a voltage controlled oscillator (VCO) with a reference frequency, and delivers voltage pulses that are a function of the phase difference between the reference frequency and the feedback frequency to an integrator, or filter.
The integrator integrates the voltage pulses received from the phase detector and then delivers the integrated output to the VCO, driving the VCO away from its free-running frequency and toward phase lock with the reference oscillator. Although it may take several or many cycles, eventually the VCO will lock onto the reference frequency, which is commonly crystal controlled.
While some phase locking oscillators are used to provide a carrier that is not modulated, the output frequencies of phase locking oscillators may be frequency modulated on an AC basis at any frequency that is higher than the natural loop frequency by applying a modulating voltage to the VCO. Alternately, the output frequency of the phase locking oscillator may be DC modulated, or both AC and DC modulated, as taught by Lautzenhiser in U.S. Pat. No. 5,091,706, issued Feb. 25, 1992; U.S. Pat. No. 5,097,230, issued Mar. 17, 1992; and U.S. Pat. No. 5,311,152, issued May 10, 1994.
When a VCO is used in a high performance electronic circuit such as a phase locking oscillator, incidental frequency modulation of the output frequency of the VCO is a critical problem, as will become apparent as this discussion continues.
A basic principle of electronic and electrical design is that any wire conductor or any trace on a circuit board that vibrates through an electrical field will induce a voltage in that wire or trace. And, of course, it is inherent in electronic circuits that there are various electrical fields.
Induced voltages, or noise, or voltage spikes that are superimposed on a voltage driving a voltage controlled oscillator, will cause unwanted modulation of output frequency. This unwanted modulation is called incidental frequency modulation. Incidental frequency modulation (IFM) is a serious problem in high performance electronic circuits.
For instance, if a given voltage controlled oscillator (VCO) has a sensitivity of 20 MHz per volt, an induced voltage of only 1.0 millivolt will cause incidental frequency modulation of 20 kHz. Since 20 kHz is the maximum allowable incidental frequency modulation for many military devices, vibration of a wire conductor or trace leading to a VCO can consume the entire allowable incidental frequency modulation bandwidth.
Of course, every attempt is made to keep wire conductors and traces short, thereby minimizing the problem of incidental frequency modulation. However, often an electrical connection must be made from some relatively distant portion of a circuit board, or even from a separate board.
Voltage controlled oscillators have many uses in addition to being a part of a phase locking loop, and whenever voltage controlled oscillators are used in high performance electronic circuits, incidental frequency modulation is potentially a serious problem.
Reducing the sensitivity (MHz per volt) of the VCO by a factor of ten would reduce incidental frequency modulation caused by induced voltages by the same factor of ten. However, the maximum frequency locking range, or capture range, that could be achieved would also be reduced by a factor of ten, in many applications preventing a phase locking oscillator from phase locking.
In summary, while prior art crystal-controlled phase locking oscillators provide improved frequency stability by minimizing both short term and long term frequency drift, voltage spikes and other electrical noise, whether emanating from the integrator, the lead connecting the integrator to the VCO, or from some other induced voltage, widen the required bandwidth, and make it difficult to manufacture electronic apparatus within military specifications.
Voltage controlled oscillators (VCOs) are provided in which incidental frequency modulation is reduced by a factor up to 10.0 or more. Therefore, phase locking oscillators using VCOs of the present invention also enjoy reduced incidental frequency modulation.
In all embodiments of the present invention, reduction in incidental frequency modulation is reduced by reducing a frequency-deviation sensitivity of the VCO. And, in all embodiments, the VCO includes means for restoring a maximum frequency range and a capture range of the phase locking oscillator.
In several embodiments, the frequency-deviation sensitivity is reduced when a frequency of a frequency-control voltage applied to a VCO is above a predetermined roll-off frequency. And the frequency-deviation sensitivity is increased, or restored, when the VCO is subjected to a frequency-control voltage whose frequency is below the predetermined roll-off frequency. In like manner, the VCO responds at the higher frequency-deviation sensitivity if the frequency-control voltage is a constant DC voltage.
The result is that, when used as a part of a phase locking oscillator, the voltage controlled oscillator responds at the reduced frequency-deviation sensitivity to loop frequencies which are above the predetermined roll-off frequency, and at full frequency-deviation sensitivity to loop frequencies, including DC, which are below the predetermined roll-off frequency.
Therefore, voltage spikes and other electrical noise are attenuated in direct proportion to the reduction in frequency-deviation sensitivity of the VCO, and undesirable incidental frequency modulation caused by the voltage spikes and other electrical noise is attenuated by the same ratio.
While the frequency deviation sensitivity of the voltage controlled oscillator is reduced in response to frequencies above the predetermined roll-off frequency, both the capture range and the frequency response of the phase locking loop remains unaffected.
In two embodiments of the VCOs of the present invention, incidental frequency modulation also is attenuated by reducing a frequency-deviation sensitivity thereof, but a maximum frequency range of the VCO is maintained by means, included in the VCO, for augmenting a frequency-control voltage applied thereto.
Therefore, all of the VCOs of the present invention include means for reducing the frequency-deviation sensitivity, whereby a maximum frequency range is also reduced, and all of the VCOs include means for restoring the maximum frequency range. When used in a phase locking oscillator the means for restoring the maximum frequency range also provides means for restoring a capture range of the phase locking oscillator that was reduced by reducing the frequency-deviation sensitivity of the VCOs.
All components of the VCOs, both for reducing the frequency-deviation sensitivity and for restoring the maximum frequency range thereof, are contained in a shielded can, thereby isolating all of their components from stray voltage fields.
By using VCOs of the present invention in phase locking oscillators, voltage spikes, other electrical noises, AC modulation signals, and incidental frequency modulation are attenuated. This is true for voltage spikes and other electrical noises developed within the phase locked loop, or introduced to the VCO from an external source.
In a first aspect of the present invention, a method is provided for reducing incidental frequency modulation of a voltage controlled oscillator in which an output frequency is driven from a free-running frequency in response to a frequency-control voltage and in proportion to both the frequency-control voltage and a frequency-deviation sensitivity, which method comprises the steps of: reducing the frequency-deviation sensitivity, whereby noise spikes, incidental frequency modulation, and a maximum frequency range for a maximum frequency-control voltage are all reduced; and restoring at least a portion of the reduced maximum frequency range without increasing the maximum frequency-control voltage.
In a second aspect of the present invention, a method is provided for reducing incidental frequency modulation of a phase locking oscillator in which an output frequency phase locks to a reference frequency by comparing a feedback frequency to the reference frequency, producing a frequency-control voltage in response to the comparing step, and controlling the output frequency in response to the frequency-control voltage, which method comprises: reducing a frequency-deviation sensitivity of the output frequency to the frequency-control voltage, whereby noise spikes, incidental frequency modulation, and a capture range are all reduced; and restoring more than one-fourth of the reduced capture range.
In a third aspect of the present invention, a method is provided for reducing incidental frequency modulation of a voltage controlled oscillator in which an output frequency is driven from a free-running frequency in response to a frequency-control voltage and in proportion to both the frequency-control voltage and a frequency-deviation sensitivity, which method comprises the steps of: reducing the frequency-deviation sensitivity to a frequency-control voltage whose frequency is above a predetermined frequency, whereby noise spikes and incidental frequency modulation are reduced; and restoring at least a portion of the reduced frequency-deviation sensitivity in response to a frequency-control voltage whose frequency is below the predetermined frequency.
In a fourth aspect of the present invention, a method is provided for reducing incidental frequency modulation of a phase locking oscillator in which an output frequency phase locks to a reference frequency by comparing a feedback frequency to the reference frequency, producing a frequency-control voltage in response to the comparing step, and controlling the output frequency in response to the frequency-control voltage, which method comprises: reducing a frequency-deviation sensitivity of the output frequency to a frequency-control voltage whose frequency is above a predetermined frequency, whereby noise spikes and incidental frequency modulation are reduced; and restoring more than one-fourth of the reduced frequency-deviation sensitivity in response to a frequency-control voltage whose frequency is below the predetermined frequency.
In a fifth aspect of the present invention, a method is provided for reducing incidental frequency modulation of a voltage controlled oscillator in which an output frequency is driven from a free-running frequency in response to a frequency-control voltage and in proportion to both the frequency-control voltage and a frequency-deviation sensitivity, which method comprises the steps of: reducing the frequency-deviation sensitivity to the frequency-control voltage, whereby noise spikes, incidental frequency modulation, and a maximum frequency range for a maximum frequency-control voltage are reduced; increasing the reduced frequency range; and the increasing step comprises proportionally combining a supplementary voltage and the frequency-control voltage, and controlling the output frequency as a function of the proportionally-combined voltages.
In a sixth aspect of the present invention, a method is provided for reducing incidental frequency modulation of a phase locking oscillator in which an output frequency phase locks to a reference frequency by comparing a feedback frequency to the reference frequency, producing a frequency-control voltage in response to the comparing step, and controlling the output frequency in response to the frequency-control voltage, which method comprises the steps of: dividing the frequency-control voltage, whereby noise spikes, incidental frequency modulation, and a capture range are reduced; increasing the reduced capture range; and the increasing step comprises supplementing the divided frequency-control voltage with a supplementary voltage, and controlling the output frequency as a function of the supplemented frequency-control voltage.
In a seventh aspect of the present invention, a voltage controlled oscillator with reduced incidental frequency modulation is provided that produces a free-running output frequency at an output frequency terminal, and whose output frequency is changed at a predetermined frequency-deviation sensitivity in response to a frequency-control voltage applied to an input voltage terminal, which comprises: means for reducing the frequency-deviation sensitivity, whereby voltage spikes, resultant incidental frequency modulation, and a maximum frequency range for a maximum frequency-control voltage are reduced; and means for restoring at least a portion of the reduced maximum frequency range without the frequency-control voltage exceeding the maximum frequency-control voltage.
In an eighth aspect of the present invention, a phase locking oscillator with reduced incidental frequency modulation is provided which comprises a phase detector, an integrator that produces a frequency-control voltage, and a voltage controlled oscillator that produces an output frequency in response to the frequency-control voltage, which comprises: means for reducing a frequency-deviation sensitivity of the voltage controlled oscillator to the frequency-control voltage, whereby voltage spikes, resultant incidental frequency modulation, and a capture range are all reduced; and means for restoring at least a portion of the reduced capture range.
In a ninth aspect of the present invention, a voltage controlled oscillator with reduced incidental frequency modulation is provided which produces a free-running output frequency at an output frequency terminal, and whose output frequency is changed at a predetermined frequency-deviation sensitivity in response to a frequency-control voltage applied to an input voltage terminal, which comprises: means for reducing the frequency-deviation sensitivity when a frequency of the frequency-control voltage is above a predetermined magnitude, whereby voltage spikes and resultant incidental frequency modulation are reduced; and means for restoring at least a portion of the reduced frequency-deviation sensitivity when the frequency of the frequency-control voltage is below the predetermined magnitude.
In a tenth aspect of the present invention, a phase locking oscillator with reduced incidental frequency modulation is provided which comprises a phase detector, an integrator that produces a frequency-control voltage, and a voltage controlled oscillator that produces an output frequency in response to the frequency-control voltage, which comprises: means for reducing a frequency-deviation sensitivity of the voltage controlled oscillator when a frequency of the frequency-control voltage is above a predetermined magnitude, whereby voltage spikes and resultant incidental frequency modulation are reduced; and means for restoring at least a portion of the reduced frequency-deviation sensitivity when the frequency of the frequency-control voltage is below the predetermined magnitude.
In an eleventh aspect of the present invention, a voltage controlled oscillator with reduced incidental frequency modulation is provided which produces a free-running output frequency at an output frequency terminal, and whose output frequency is changed at a predetermined frequency-deviation sensitivity in response to a frequency-control voltage applied to an input voltage terminal, which comprises: means for reducing the frequency-deviation sensitivity, whereby voltage spikes, a maximum frequency range for a maximum control voltage, and incidental frequency modulation are reduced; means, being interposed intermediate of the input voltage terminal and the output frequency terminal, for increasing the reduced frequency range; and the means for increasing the reduced frequency range comprises means for proportionally combining an other voltage with the frequency-control voltage.
In a twelfth aspect of the present invention, a phase locking oscillator with reduced incidental frequency modulation is provided which comprises a phase detector, an integrator that produces a frequency-control voltage, and a voltage controlled oscillator that produces an output frequency in response to the frequency-control voltage, which comprises: means for dividing the frequency-control voltage, whereby a capture range and incidental frequency modulation are reduced; means, being interposed intermediate of an input voltage terminal and an output frequency terminal of the voltage controlled oscillator, for increasing the reduced capture range; and the means for increasing the reduced capture range comprises means for proportionally combining another voltage with the divided frequency-control voltage.
In a thirteenth aspect of the present invention, a method is provided for reducing incidental frequency modulation of a phase locking oscillator, which method comprises: reducing a frequency-deviation sensitivity of the phase locking oscillator in response to frequency-control voltages having frequencies above a predetermined frequency; and restoring at least a portion of the reduced frequency-deviation sensitivity in response to frequency-control voltages having frequencies below the predetermined frequency.